Assimilation of Planets by Red Giant Stars

The typical red giant star rotates slowly. This characteristic is expected from the conservation of angular momentum as these stars expand during their evolution. Nevertheless, a small percentage of red giant stars are rapidly rotating. One possible source of these stars' excess angular momenta is the orbital angular momentum of a planetary companion. The transfer of orbital angular momentum to the stellar envelope decays the planet's orbit, ultimately leading to the rapid in-spiral of the planet into the star. Using the known sample of exoplanets around main sequence host stars, I simulated both the future evolution of these stars and the expected interactions with their planets and found that Jupiter-mass planets residing at inner solar system distances-relatively common in exoplanetary systems-can contribute enough angular momentum to cause rapid rotation in their host stars during the red giant phase. Gas giant planets are also massive enough to alter the chemical composition of their host stars' envelopes when they are accreted. The central experiment of this thesis is to search for abundance anomalies in the rapid rotators that could be indicative of planet accretion. Hypothetical anomalies include the replenishment of light elements that are diluted by giant stars during first dredge-up (such as the stellar surface abundance of lithium), changes in isotopic abundance ratios that were altered by nucleosynthesis (such as increasing the stellar surface 12 C/ 13 C), and the preferential enhancement of refractory elements (indicative of the accretion of chemically fractionated material such as a planet). To increase the total number of known rapid rotators, I measured rotational velocities in a large database of spectra collected for the Grid Giant Star Survey developed for NASA's Space Interferometry Mission's astrometric grid. The 28 new rapid rotators discovered in this sample were combined with rapid rotators from the literature and a control sample of slow rotators to form iii a new sample for the abundance experiment. This thesis presents evidence that the accretion of planets of a few Jupiter masses with "normal" planetary compositions can reproduce both the observed rotational velocities and abundances of red giant rapid rotators.

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